Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as mp3 players, are in widespread use. Performance of such devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events.
Because the acoustic environment around personal audio devices, such as wireless telephones, can change dramatically, depending on the sources of noise that are present and the position of the device itself, it is desirable to adapt the noise canceling to take into account such environmental changes. For example, many adaptive noise cancelling systems utilize an error microphone for sensing acoustic pressure proximate to an output of an electro-acoustic transducer (e.g., a loudspeaker) and generating an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer. When the transducer is close to a listener's ear, the error microphone signal may approximate the actual acoustic pressure at a listener's eardrum (a location known as a drum reference point). However, because of the distance between the drum reference point and the location of the error microphone (known as the error reference point) the error microphone signal is only an approximation and not a perfect indication of acoustic pressure at the drum reference point. Thus, because noise cancellation attempts to reduce ambient audio sounds present in the error microphone signal, performance of a noise cancellation system may be the greatest when the distance between the drum reference point and the error reference point is small. As the distance increases (e.g., transducer held against the ear at a lower pressure), the performance of the noise cancellation system may degrade, partly because the gain of the transfer function from the error reference point to the drum reference point decreases with such increased distance. This degradation is not accounted for in traditional adaptive noise cancellation systems.